Filter



F. J. GIVEN May 7, 1935.

FIG.

5 2 F F U f P a 6 0 l 5 lNVEA/TOR F. J.G/VEN 8V ATTORNEY Patented May 7, 1935 FILTER Frederick J. Given, East Orange, N. .I., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 6, 1934, Serial No. 733,978

This invention relates, in general, to filters and more particularly to a filter for suppressing radio interference.

In-an automatic telephone system, when a sub- 6 scriber operates his calling dial, a contact is ordinarily made and broken alternately which causes the direct current supply from the central oifice battery to become a pulsating current for operating the switching mechanism in the central ofiice. The build-up and decay of the pulsating current is sufilciently fast to build up to its maximum value at a much faster rate than the pulsating frequency. When the contact is broken, the current falls very rapidly and since the frequency components are within the radio range, the circuit has a tendency to function as a radio frequency generator and the high frequency currents generated radiate from the winding of the telephone set causing interference in nearby radio receivers.

The induction resulting from opening the circuit of the dial contacts is due principally to radio frequency oscillations set up by the charging and discharging of the small capacities formed by the wiring of the apparatus and since this capacity is very small voltages are built up in the circuit sufilciently high to break down and discharge across the contact points at a high frequency. The induction caused on closure of the dial contacts is largely due to radio frequency components of the square top wave produced by the sudden short circuit of the contacts.

The principal object of the invention is, therefore, a filter which will efllciently suppress high frequency curents set up by the operation of contacts in an electrical circuit. A further object of the invention is such'a filter which can be manufactured economically and which is small in size so that it can be installed in restricted spaces such as in a telephone desk stand base.

In accordance with the features of this invention, these objects are attained by means of a combined unit having the desired capacity, inductance and effective resistance values, and one which may be manufactured by a single winding operation. In this filter a pair of conductors are wound in parallel to obtain a capacity element of the desired value and one of these conductors is extended so that the additional turns provide a series inductance of the desired value. By then short-circuiting some of the turns of this conductor, an effective resistance is introduced 55 which, at radio frequencies, is equivalent to that obtained by a resistance in parallel with themductance element. g

These and other features of the invention may be more clearly understood by reference to the accompanying drawing in which,

Fig. 1 is a perspective view of a filter unit embodying the features of the invention;

Fig. 2 shows the manner in which the foils and terminals are arranged in constructing the filter of Fig. 1;

Fig. 3 shows the schematic circuit of the filter; and

Fig. 4 shows the equivalent typical filter structure.

Referring now more particularly to Figs. 1 and 2, the combined unit consists of the conductors l0 and II of metallic foil, such as tinfoil or aluminum foil alternating with dielectrics l2--I2 of paper or similar material wound as shown into I the form of a coil I3. The metallic sheet II is of greater length than the sheet [0 and is provided with terminals l4 and i5 at opposite ends and with terminal [8 which is substantially in line with the outer end of the metallic sheet Ill. The portion of this foil between terminals l5 and i6 and one of the paper dielectrics is wound on the outer portion of the coil to form an inductance unit, and the portion of foil I l between terminals l4 and I6 cooperates with the foil 10, to form the capacity element of the combined unit. The terminals I 4 and I6 are connected together by means of conductor I! to form, in effect, a closed secondary winding, the resistance of this conductor being preferably low so that the winding is practically short-circuited. Terminal I 8 is connected to substantially the midpoint of the metallic sheet l0. Terminal leads 20, 2| and 22 are connected respectively to terminals it, IS and I8.

By connecting the terminals l4 and It with a conductor of low resistance which results in practically short-circuiting the turns of this portion of the unit, a filter circuit is obtained as illustrated in Fig. 3, this filter circuit being connected across the dial contacts 23, 24 as shown. The capacity between the foil l0 and the portion of foil ll between terminals l4 and I6, is used as the shunt capacity 25 of the filter. The portion between terminals l5 and i6 is used as the series inductance 26. Due to the. structure, the portions of the wound foil ll between terminals l5 and I6 and between terminals l4 and i6 form two inductance elements 26 and 21 which are inductively coupled together. By short-circuiting the portion between terminals It and It by means of conductor I1, the same efieot is attained as if a resistance 28 were connected in shunt to the inductance 26 between terminals l5 and Hi. This is shown by the equivalent filter circuit of Fig. 4. The value of the efiective resistance depends, of course, on the coupling between the two inductances, the ratio of the turns and the total resistance of the inductance 21 and the conductor connecting its terminals.

The filter, as thus described, is made up in a single winding operation and thus manufactured very economically, and it is compact which permits its being installed in restricted spaces, such for example, in the base of a standard type desk set. Furthermore, its selective characteristics are such that it operates efliciently to suppress currents of radio frequency resulting from the operation of the dial contacts and does not cause excessive burning or wear of the contacts themselves.

As described and shown on the drawing the series inductance element is on the outer portion of the filter, but it is, of course, obvious that the foils could be wound in the opposite direction so that this inductance would be in the center of the filter.

What is claimed is:

1. A filter comprising a pair oi conducting sheets insulated from each other and wound in parallel to form a capacity element, one oi said sheets having an extended portion wound on itself to form an inductance element and having a portion of its turns short-circuited to produce an eflfective resistance in shunt with the inductive element.

2. A filter comprising a pair of metallic sheets separated by a sheet of dielectric and wound into the form of a coil, one of said metallic sheets being of greater length and having its outer portion wound on itself to form an inductive element and having its inner portion cooperating with the other metallic sheet to form a capacity element, and means for short-circuiting a portion of said metallic sheet of greater length to produce an effective resistance in parallel with the inductive element.

3. A filter comprising a coiled conductor wound to form an inductance element, a second conductor coiled in parallel with a portion of the first conductor to form a capacity element with said portion and a conductor connecting the terminals of said portion of the first conductor to form an inductance coupled to the other portion of said first conductor, whereby said other portion is effectively shunted by a resistance.

FREDERICK J. GIVEN. 

